Residual fractions obtained from the distillation of poor quality crudes contain substantial amounts of metals such as Ni, V, Fe, Cu, and Na, and have a high concentration of asphaltenes, polynuclear aromatics and other coke precursors. These residual oils are rendered more suitable as feedstocks to refinery processing units, such as fluid catalytic cracking and hydrocracking, by first pretreating the residual oils, in the absence of hydrogen, in order to remove substantial portions of the metal and coke precursor contaminants. Such pretreatment processes involve contacting poor quality, high boiling residual oils with a solid sorbent particle exhibiting relatively low or no significant cracking activity, under conditions of time, temperature and pressure sufficient to reduce the metals and Conradson carbon residue values of the residual oil feed to within more acceptable limits for downstream processing such as catalytic cracking.
The literature suggests many processes for the reduction of metals and coke precursors in residual and other contaminated oils in the absence of added hydrogen. One such process is described in U.S. Pat. Nos. 4,243,514; 4,263,128; 4,311,580; 4,328,091 and 4,427,538, assigned to Engelhard Minerals and Chemicals Inc., which patents are incorporated herein by reference. Other examples are U.S. Pat. No. 4,427,539, assigned to Ashland Oil, Inc., and U.S. Pat. No. 3,983,030, assigned to Mobil Oil Corporation.
During residual oil pretreatment processes, the contaminant metals may be deposited in a relatively nonvolatile form on inert solids. These metal contaminants are generally specified as parts per million (ppm) nickel equivalents, defined as the sum of the nickel content in ppm plus one-fifth the vanadium content in ppm, plus one-tenth the iron content in ppm or (nickel +0.2 vanadium+0.1 iron). The deposition of these contaminant metals on the inert solid sorbents during the pretreatment process poses two problems. First, it has been found that as the vanadium content of the sorbent increases above 5000 ppm, the sorbent begins to have fluidization problems. More importantly, the elevated temperatures found in the regeneration zones, which are necessary for the removal of carbonaceous deposits, cause significant amounts of vanadium oxides (vanadia) to melt, flow and form a liquid coating on the sorbent particles, rendering the particles less effective for sorption, while also interrupting fluidization. This aspect of the problem, related to deposited vanadium on the inert sorbent, is taught in U.S. Pat. No. 4,469,588, which is incorporated herein by reference.
A second problem associated with the deposition of contaminant metals on inert sorbents is the dehydrogenation activity of the deposited metals. Nickel, and to a lesser extent, vanadium, iron and copper, can themselves promote undesirable dehydrogenation reactions which result in excessive production of hydrogen. This excess hydrogen will tend to overload the gas recovery system and produce excessive coke, resulting in poorer yields of liquid products, and excessively high temperatures during regeneration of the carbonaceous materials on the inert solids.
Previously, the problems caused by the deposition of contaminant metals on inert sorbents during heavy oil pretreatment have been overcome by removal of a portion of spent sorbent inventory on a continuous basis, and replacing it with fresh, metal contaminant-free sorbent.
Recently, however, inventions have been disclosed that seek to mitigate the deposited metals-induced problems via means other than, or in addition to, simply replacing a portion of the spent sorbent inventory. Solutions to the first problem, namely the vanadium-induced pore plugging of the sorbent, have been proposed in U.S. Pat. Nos. 4,469,588, 4,513,093 and 4,515,900, assigned to Ashland Oil, Inc. The inventions involve the introduction of metal additives onto the sorbent material to immobilize vanadia by forming compounds, complexes or alloys between the deposited vanadia and the additive material. These newly formed compositions have a higher melting point than the temperatures permitted to be achieved during the regeneration operation. The list of additive metals include Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf, Nb, Ta, Mn, Tc, Re, Ni, Ru, Rh, Pd, Os, Ir, Pt, In, Tl, Bi, As, Sb, and all elements in the lanthanide and actinide series. The additives are chosen such that, based on a 1:1 mole ratio of the metal additive oxide to vanadium pentoxide, the melting points of the mixtures are greater than regeneration temperatures. No mention is made in these references of a process to mitigate the dehydrogenation effects of deposited nickel. In fact, nickel is claimed as one of the additives for immobilizing vanadia on the sorbent.
U.S. Pat. No. 4,325,809, assigned to Engelhard Minerals and Chemicals Inc., teaches the introduction of a silica donor along with the sorbent material, reacting the mixture at high temperature in the presence of steam to induce migration of silica from the donor to mask metal on the sorbent. Again, this invention does not disclose a method for passivating the dehydrogenation activity of the contaminant metals deposited on the inert sorbent.
Thus, among other factors, it is an object of the present invention, to passivate the dehydrogenation activity of deposited metals, primarily nickel, and to a lesser extent, vanadium, iron, sodium and copper, on inert, solid sorbents during non-hydrogenative pretreatment of heavy oil. This passivation comprises the deposition on said inert, solid sorbent of bismuth or compounds of bismuth.